1. Field of the Invention
The invention relates generally to the continuous casting of metal. It relates more particularly to the casting of thin carbon-steel strip on plants of the type known as xe2x80x9ctwin-roll castersxe2x80x9d.
2. Discussion of the Background
Considerable progress has been made in recent years in the development of processes for casting thin steel strip directly from liquid metal. The process mainly used at the present time is the casting of said liquid metal between two internally cooled rolls, rotating about their horizontal axes in opposite directions and placed opposite each other, the minimum distance between their surfaces being approximately equal to the thickness that it is desired to confer on the cast strip (for example a few mm). The casting space containing the liquid steel is defined by the lateral surfaces of the rolls, on which surfaces the strip starts to solidify, and by side closure plates made of refractory, which are applied against the ends of the rolls. The liquid metal starts to solidify on contact with the outer surfaces of the rolls, on which surfaces solidified xe2x80x9cshellsxe2x80x9d form, arrangements being made for these shells to meet in the region of the xe2x80x9cnipxe2x80x9d, that is to say the region where the distance between the rolls is a minimum.
These casting processes are used both for casting carbon steel and for casting stainless steels or other ferrous alloys. However, the industrial application of twin-roll casting to carbon steels cannot be envisaged in an acceptable manner unless it is possible to obtain, in a constant manner, a strip surface quality sufficient for the subsequent treatments undergone by this strip (cold rolling operations, surface treatments, etc) to be possible and to result in the formation of products free of unacceptable defects. It is, in particular, of paramount importance for the strip produced by a twin-roll casting plant to be free of surface cracks, called crazes, because otherwise serious incidents may occur during its cold rolling.
In order to try to avoid such crazing, solutions have already been proposed which involve giving the surface of the casting rolls a particular texture, namely a succession of contiguous and parallel grooves, preferably combined with a silicon-manganese killed cast metal having a high sulfur content, of greater than 0.02% (document EP-A-0,740,972). However, this solution complicates the preparation of the rolls compared with the more conventional methods of preparation and, under its optimum conditions, limits the field of application of the cast products to those in which the high sulfur content of the metal is tolerable.
One object of the invention is to provide a process for manufacturing thin carbon-steel strip by twin-roll casting, resulting in the reliable production of strip free of surface crazing, this method having neither to limit the field of application of the products nor to necessarily lead to the use of long and complex methods for preparing the surface of the rolls. The apparatus involved and the products produced are also objects of the invention.
In accord with the objects, one subject of the invention is a process for manufacturing carbon-steel strip having a thickness of less than or equal to 10 mm directly from liquid steel, by casting said liquid steel between the lateral surfaces, made of copper or copper alloy, of two internally cooled rotating horizontal rolls, wherein preferably:
said liquid steel has the composition, in percentages by weight based on total weight: carbonxe2x89xa60.5%, manganese from 0.2 to 2%, siliconxe2x89xa62%, the %Mn/%Si ratio being between 3 and 16, and optionally aluminum+titanium+zirconiumxe2x89xa60.10%, and containing of course iron and the usual impurities;
said lateral surfaces of the rolls have contiguous dimples, giving said surfaces a roughness Rz of between 40 and 200 xcexcm and a roughness Ra of between 10 and 40 xcexcm; and
the atmosphere surrounding the meniscus of the liquid steel present between the rolls contains between 40 and 100% nitrogen, the balance preferably being composed of an inert gas insoluble in the liquid steel or of a mixture of such inert gases.
As noted, the invention also relates to strip produced by this process, as well as to the casting rolls necessary for its practical implementation.
Generally speaking, the invention indues combining particular conditions regarding the composition of the steel, the surface finish of the rolls and the composition of the atmosphere surrounding the surface of the liquid steel present between the rolls. The inventors have found that such a combination results in the uniform production of products having the required surface quality. The conditions under which this process is preferably carried out also have the advantage of not resulting in a casting process substantially more restrictive than the usual processes. In addition, the process of the invention requires that no action be taken on the composition of the metal, which would appreciably increase the cost of the strip, since it is not necessary to add expensive alloying elements to the liquid metal or to impose on the liquid metal unusually low contents of certain elements. Likewise, the ranges of use of products manufactured from the strip thus produced are not limited by the presence, in amounts higher than is customary, of elements such as sulfur, which could radically impair the mechanical properties of said products. Finally, this invention does not require increasing the heat flux to be extracted from the liquid metal by the rolls, whereas such an increase could be damaging to the service life of the cooled external surface of said rolls.